1. Field of the Invention
The present invention relates generally to the field of lithography. More specifically, the present invention relates to cleaning surfaces in a lithographic projection apparatus.
2. Description of the Related Art
The term “patterning device” or “patterning structure” as here employed should be broadly interpreted as referring to a device or structure that can be used to endow an incoming radiation beam with a patterned cross-section, corresponding to a pattern that is to be created in a target portion of the substrate; the term “light valve” can also be used in this context. Generally, the said pattern will correspond to a particular functional layer in a device being created in the target portion, such as an integrated circuit or other device (see below). Examples of such patterning means include:                A mask. The concept of a mask is well known in lithography, and it includes mask types such as binary, alternating phase-shift, and attenuated phase-shift, as well as various hybrid mask types. Placement of such a mask in the radiation beam causes selective transmission (in the case of a transmissive mask) or reflection (in the case of a reflective mask) of the radiation impinging on the mask, according to the pattern on the mask. In the case of a mask, the support structure will generally be a mask table, which ensures that the mask can be held at a desired position in the incoming radiation beam, and that it can be moved relative to the beam if so desired.        A programmable mirror array. One example of such a device is a matrix-addressable surface having a viscoelastic control layer and a reflective surface. The basic principle behind such an apparatus is that (for example) addressed areas of the reflective surface reflect incident light as diffracted light, whereas unaddressed areas reflect incident light as undiffracted light. Using an appropriate filter, the said undiffracted light can be filtered out of the reflected beam, leaving only the diffracted light behind; in this manner, the beam becomes patterned according to the addressing pattern of the matrix-addressable surface. An alternative embodiment of a programmable mirror array employs a matrix arrangement of tiny mirrors, each of which can be individually tilted about an axis by applying a suitable localized electric field, or by employing piezoelectric actuation means. Once again, the mirrors are matrix-addressable, such that addressed mirrors will reflect an incoming radiation beam in a different direction to unaddressed mirrors; in this manner, the reflected beam is patterned according to the addressing pattern of the matrix-addressable mirrors. The matrix addressing can be performed using suitable electronic means. In both of the situations described hereabove, the patterning means can comprise one or more programmable mirror arrays. More information on mirror arrays as here referred to can be gleaned, for example, from U.S. Pat. No. 5,296,891 and U.S. Pat. No. 5,523,193, and PCT patent applications WO 98/38597 and WO 98/33096, which are incorporated herein by reference. In the case of a programmable mirror array, the said support structure may be embodied as a frame or table, for example, which may be fixed or movable as needed.        A programmable LCD array. An example of such a construction is given in U.S. Pat. No. 5,229,872, which is incorporated herein by reference. As above, the support structure in this case may be embodied as a frame or table, for example, which may be fixed or movable as needed.For purposes of simplicity, the rest of this text may, at certain locations, specifically direct itself to examples involving a mask and mask table; however, the general principles discussed in such instances should be seen in the broader context of the patterning means as hereabove set forth.        
Lithographic projection apparatus can be used, for example, in the manufacture of integrated circuits (ICs). In such a case, the patterning structure may generate a circuit pattern corresponding to an individual layer of the IC, and this pattern can be imaged onto a target portion (e.g. comprising one or more dies) on a substrate (silicon wafer) that has been coated with a layer of radiation-sensitive material (resist). In general, a single wafer will contain a whole network of adjacent target portions that are successively irradiated via the projection system, one at a time. In current apparatus, employing patterning by a mask on a mask table, a distinction can be made between two different types of machine. In one type of lithographic projection apparatus, each target portion is irradiated by exposing the entire mask pattern onto the target portion in one go; such an apparatus is commonly referred to as a wafer stepper. In an alternative apparatus—commonly referred to as a step and scan apparatus—each target portion is irradiated by progressively scanning the mask pattern under the projection beam in a given reference direction (the “scanning” direction) while synchronously scanning the substrate table parallel or anti parallel to this direction; since, in general, the projection system will have a magnification factor M (generally <1), the speed V at which the substrate table is scanned will be a factor M times that at which the mask table is scanned. More information with regard to lithographic devices as here described can be gleaned, for example, from U.S. Pat. No. 6,046,792, incorporated herein by reference.
In a manufacturing process using a lithographic projection apparatus, a pattern (e.g. in a mask) is imaged onto a substrate that is at least partially covered by a layer of radiation sensitive material (resist). Prior to this imaging step, the substrate may undergo various procedures, such as priming, resist coating and a soft bake. After exposure, the substrate may be subjected to other procedures, such as a post exposure bake (PEB), development, a hard bake and measurement/inspection of the imaged features. This array of procedures is used as a basis to pattern an individual layer of a device, e.g. an IC. Such a patterned layer may then undergo various processes such as etching, ion implantation (doping), metallization, oxidation, chemo mechanical polishing, etc., all intended to finish off an individual layer. If several layers are desired, then the whole procedure, or a variant thereof, will have to be repeated for each new layer. Eventually, an array of devices will be present on the substrate (wafer). These devices are then separated from one another by a technique such as dicing or sawing, whence the individual devices can be mounted on a carrier, connected to pins, etc. Further information regarding such processes can be obtained, for example, from the book “Microchip Fabrication: A Practical Guide to Semiconductor Processing”, Third Edition, by Peter van Zant, McGraw Hill Publishing Co., 1997, ISBN 0 07 067250 4, incorporated herein by reference.
For the sake of simplicity, the projection system may hereinafter be referred to as the “lens”; however, this term should be broadly interpreted as encompassing various types of projection system, including refractive optics, reflective optics, and catadioptric systems, for example. The radiation system may also include components operating according to any of these design types for directing, shaping or controlling the projection beam of radiation, and such components may also be referred to below, collectively or singularly, as a “lens”. Further, the lithographic apparatus may be of a type having two or more substrate tables (and/or two or more mask tables). In such “multiple stage” devices the additional tables may be used in parallel, or preparatory steps may be carried out on one or more tables while one or more other tables are being used for exposures. Dual stage lithographic apparatus are described, for example, in U.S. Pat. No. 5,969,441 and WO 98/40791, incorporated herein by reference.
Contamination on the surface of elements, such as optical elements like reflectors, lenses, deflectors, sensors or the patterning structure, degrades the performance of the apparatus. This is especially true for apparatus employing EUV radiation, which is especially sensitive to intensity loss, but is also a problem for Deep UV (DUV), electron (EPL) and ion projection lithography (IPL). Contamination may, for example, follow from hydrocarbon contaminants in the apparatus being deposited on the surfaces. Radiation (especially EUV radiation) incident on the surface breaks the bonds of the hydrocarbons to form chemical bonds between carbon atoms from the hydrocarbons and the surface, leaving an amorphous carbon layer. The hydrocarbons themselves can also degrade the performance of the apparatus. Other contaminants include O2 and H2O and crystals of refractory compounds, including those that contain sulfates, phosphates or silanes. An amorphous carbon layer, for example, absorbs a significant fraction of the EUV radiation subsequently incident on an optical surface (approximately 1% in a 1 nm thick layer, for example). Therefore, for an apparatus with ten reflectors, (which even when perfectly clean, are not efficient reflectors), each with an amorphous carbon layer formed on its surface, the intensity of the EUV beam at the substrate will be reduced by a further 10%. This in turn increases the exposure time for each substrate and reduces the throughput of the apparatus.
For sensors, the absorption of the EUV radiation prohibits calibrated dose measurements and reduces the signal-to-noise ratio. Sensors are needed to accurately measure the intensity of EUV radiation in an EUV apparatus in order to ensure that the appropriate exposure dose is provided to a substrate. It is therefore necessary for the contamination to be removed without damaging the sensitive diode.
Under the normal conditions of an EUV apparatus, the carbon growth rate is approximately 1-10 nm/hr. Therefore, the optical elements, for example, require regular cleaning. For the cleaning to be practical, it should be performed in-situ, be completed quickly, and should not damage the optical surface.
A presently known method of removing the carbon contamination combines UV radiation and ozone to react with the carbon to form CO and CO2, which desorb from a surface and are subsequently pumped away. However, for efficient UV/ozone cleaning, a pressure of at least 1 mbar is needed. This is not compatible with an EUV lithographic apparatus in which the pressure is approximately 10−7 to 10−2 mbar. Furthermore, the cleaning rate for UV/ozone cleaning is approximately 1 to 10 nm/hr. This is approximately the same as the rate of contamination. Therefore, 50% of the operational time would be needed for cleaning. This loss of throughput is not acceptable in practical terms.
An alternative well-known cleaning method is conventional oxygen plasma cleaning. Again, oxygen radicals react with the carbon to form CO and CO2 molecules which desorb from the surface and can subsequently be pumped away. However, conventional oxygen plasma cleaning is also unsuitable for use in an EUV lithographic apparatus. A pressure of approximately 1 mbar is needed to generate a stable plasma. A plasma is also difficult to confine to a specific region and, if used in a lithographic apparatus, may damage other sensitive elements of the apparatus such as the electronics and cables.